Means for continuous fabrication of graded junction transistors



I. HARON MEANS FOR CONTINUOUS FABRICATION OF Oct. 3, 1961 c.

GRADED JUNCTION TRANSISTORS 2 Sheets-Sheet 1' Filed Oct. 22, 1956 IINVENT OR Gar/ Haron BY z ' ATTORNEYS Oct. 3, 1961 c. I. HARON A MEANSFOR commuous FABRICATION OF GRADED JUNCTION TRANSISTORS 2 Sheets-Sheet 2Filed Oct. 22, 1956 x dumb m bmxa On kwllbl INVENTOR Gar/ HaronATTORNEYS 3 002,921 BEANS FUR CONTENUOUS FABRICATION F GRADED JUNCTEONTRANSHSTQRS Carl E. Heron, Dallas, Ten, assignor to Texas Instrumentsincorporated, Dallas, Tera, a corporation of Delaware Filed Oct. 22,1956, Ser. No. 617,330 9 Claims. (Cl. 23--273) This invention relates tothe art of producing semiconductive bodies, and more particularly to anovel apparatus for use in conjunction therewith.

As is well known to those skilled in the art, semiconductor bodies whichincludes contiguous portions of differing electrical conductivity may beformed by withdrawing a partially immersed seed orystalfrom a moltenmass of semiconductor material, such as silicon or germanium. Suchsemiconductor material is extremely sensitive to thermal or mechanicaldisturbances occurring during the solidification period which accompanysuch withdrawal from the molten melt. This sensitivity renders possiblethe fabrication of graded-junction type crystals.

More particularly, it has been discovered that a change in conditions,such as an abrupt increase in the tensile force on the crystalcolumn, ora sudden change in the growth rate will produce a sharp variation in theconductivity of the orystallizing material. This phenomena has beenexploited in the present invention to form crystals having an interface,or junction with a conductivity different from the contiguous material.Moreover, by practicing the teaching of the present invention,graded-junction type semiconductor bodies can be fabricated in asubstantially continuous process.

The graded junctions can be readily formed by a melt-back process, inwhich a minute portion of the crystallized material is dropped back orotherwise reimmersed in the molten material for a predeterminedinterval. Alternatively, the liquid level of the molten semiconductormaterial may be periodically raised by small amounts to achieve exactlythe same result. In the manufacture of graded-junction germanium andsilicon transisters according to the present invention, a melt-backprocess which included reimmersing the rod to a depth of .010" forapproximately 30 seconds yielded excellent results.

After the formation of the desired junction by such a melt-back process,the gradual withdrawal of the crystalline rod is resumed, and the degreeof conductivity along the ,rod reverts to its former value. Thecontinuously alternated withdrawals and reimmersions contemplated by theinvention provide an elongated rod-like'semiconductor with a series ofgraded junctions spaced along its length. graded-junction semiconductorscontinuously instead of by the use of the conventional batch process.

According to the continuous process disclosed in this specification,semiconductor crystals are grown from a continuously replenished supplyof molten melt. replenishment of the melt by novel techniques to bedisclosed eliminates many disadvantages associated with the depletion ofthe liquid phase or molten semiconductor which characterizes theconventional batch process. Many of the disadvantages which characterizeprior art methods of'using the batch process are attributable to thedepletion of the molten mass which occurs during withdrawal of the seedcrystal. As the liquid phase of certain semiconductor materials, likegermanium, is progressively de- It is an object of the invention to formthe The.

pleted during the growth of the solid phase by the crystallizationprocess, the concentration of impurities in the remaining liquid tendsto increase. Consequently, the conductivity characteristics of a largecrystal tends to vary progressively along the length of the crystal,unless the depletion of the molten batch is continually compensated forby an operator. In the present invention, the conembodiment of theinvention.

idzgfi Patented Oct. 3, 19 61 tinually changing conditions occurring inthe feed crucibles are eliminated as a limiting factor in the crystalgrowth.

Additionally, as earlier described, means are disclosed for continuouslyforming an elongate crystalline semiconductor provided with a series ofgraded junctions spaced at substantially equal intervals. Thetemperature and mass of the molten semiconductor material, as well asgrowth rate of crystallization, are maintained substantially constantand continuous. By providing a more or less continuous makeup to thefeed crucibles, in order to avoid progressive variation in theconductivity between the graded junctions, a primary goal of theinvention is accomplished. The manner in which this goal of crystalwithdrawal from a constant mass is accomplished, in order to eliminatethe undesirable changes in conductivity associated with prior art batchprocesses will become evident as the detailed description of theinvention proceeds.

Accordingly, therefore, a primary object of this invention is to providean apparatus for continuous formation of graded-junction type crystals,accompanied by a degree of replenishment of the molten semiconductormaterial which is adeouate to eliminate undesirable variations in theconductivity of the crystal during the growing stage.

Still another object of this invention is to teach a system formaintaining substantially constant the temperature and mass of themolten semiconductor material as well as the rate of crystallization inorder to obviate adverse etiects caused by progressive depletion ofimpurities in the molten semiconductor material.

Another object of this invention is to provide a novel means for formingan extended semiconductor crystal with a plurality of graded typejunctions transversely disposed along the long axis thereof.

A further object of this invention is to disclose a novel apparatus forcontinuously forming a series of graded ty-pe junctions along anelongated crystal while simultaneously replenishing depletions in thequantity of molten semiconductor material from which the crystal isformed.

These and other objects of the present invention will become evident byreference to the following detailed description and drawing, in whichlike numerals indicate like parts and in which:

FIGURE 1 shows a cross sectional view of a preferred FIGURE 2 showsdiagrammatically a system for automatically replenishing the supply ofmolten semiconductor material.

Turrn'ng now to the drawing, and more particularly to FIGURE 1 thereof,the reference numeral 1 indicates generally a portion of the novelapparatus for accomplishing continuous growth of the elongatedcrystalline rod 2.

More particularly, a housing 3 is provided in order to provide a thermalbarrier and inhibit heat loss from the molten semiconductor material Thehousing 3 may be composed of a plurality of continguous adjacent laminaof asbestos and metal. The heat resistant qualities of the asbestos arethus exploited in combination with the reflective properties of themetal laminations.

The metallic lamina within housing 3 also act as a shield for theoscillatory radiant energy which is generated by the radio frequencyinduction coils 5 provided within the housing. The coils 5 are employedto generate internal heat and reduce the semiconductor material 4 to amolten state.

In order to confine and channel the movement of the molten material 4, afeeding crucible 6 composed of graphite or other heat resistant materialis provided within the housing 3. The upper walls of the feedingcrucible 6 engage the walls of a growth crucible 7. The growth crucible7 may be-fabricated of graphite or other equally suitable heat resistantmaterial.

The passage of molten semiconductor 4 between the outer portion of thefeeding crucible 6 and the inner portion which communicates with thegrowth crucible 7 is regulated by means of a pair of plug valves 8A and8B. These valves may be formed of graphite or the like, with a suitablefeed orifice provided therethrough. Moreover, the valves 8A and 8B arerotatably disposed within the walls of the feed crucible 6, and may beadjusted to control the rate at which molten semiconductor materialenters the growth crucible. The arrangement is such that the portions ofthe feed crucible 6 without the plug valves 8A and 8B can hold dilierentcharge materials. Even in these circumstances it is possible to have anuninterrupted change in the basic constitutent material during thecrystal growing process. There may be a dead pull during purging of onebasic material by the other, but the changeover will be continuous.

At the outermost portions of the feed crucible 6, there are providedcharge pots QA and 9B. A supply of solid granular semiconductor materialis contained within the charge pots 9A and 9B. The method and means bywhich the solid granular material in the charge pots is replenished willbe explained in connection with FIGURE '2 of this specification. Therate at which this solid material is allowed to enter the interior ofthe feed crucible 'is governed by means of a pair of adjustable barrelvalves identified by the reference characters 10A and MB, respectively.Each of the barrel valves 10A and 103 may be composed of stainless steelor other equally suitable wear resistant material. The valves aremachined in cylindrical form, and are each provided with a pair ofrecesses or pockets diametrically disposed so that the valves NA and 1GBseal off the feed crucible even when introducing material into it.

Returning now to the constructional details of the apparatus adjacentthe growth crucible 7, there is shown a cylindrical member 11 made ofquartz concentrically enclosing the crucible and associated inductioncoils. At the upper boundary of the cylindrical member 11, there isprovided a rubber end seal 13. Below the end seal 13, three separatecooling zones are defined by the transverse seals M-A, 14B and 14C,respectively. The seals 14A, 14B and 140 may be composed of temperatureresistant rubber, or like material, characterized by the ability towithstand the elevated temperatures associated with the withdrawal ofthe crystalline work product from a molten melt. In order to shield thetransverse seal 14C from the elevated ambient temperatures existingdirectly therebeneath, ring member 15 composed of monel and acting as aradiant shield is disposed across the bore of the cylindrical member 11.

During passage through the coolant zones defined by the three transverseseals above described, heat may be abstracted from the elongated crystal2 by means of convection contact with a suitable gas. For instance, acontinuous flow of helium gas, or the like, may be directed into heatexchange relationship with the portion of the upwardly rising crystalenclosed by the transverse seals. Since conventional structures andcomponents for gas cooling crystal growing apparatus are commerciallyavailable, no detailed exposition of these systems has been provided inthe present specification.

The upward tractive force required to form a continuous elongatedcrystal is supplied by a first pneumatic clutch 16 showndiagrammatically directly above the cylindrical member 11. The clutch 16is provided with a plurality of radially displaceable jaws 16a, 16b and16c.

Vertical rectilinear motion as well as rotary motion is imparted to thecrystal 2 by means of the pneumatic clutch 16. Clutch 16 is utilized tosubject the elongated crystal to an upwardly disposed tensile force aswell as 'to'a spinning action, at the start of each growth cycle.

Directly above the first pneumatic clutch 16, a second 8,002,821 I, w r"I pneumatic clutch 17 is diagrammatically illustrated. The clutch 17 isprovided withplural jaws 17a and 17b adapted to engage the crystal 2. lngeneral, the jaws 17a and 17b of the upper clutch will be retractedduring the period that the first clutch 16 is lifting the crystal rodvertically upward. However, during those intervals when the first clutch16 disengages and descends vertically in order to re-engage the crystal2 for the next upward pull, the upper clutch 17 engages the rod andcontinues the spinning and whatever other motion is required untilclutch 16 reengages the rod at which time clutch 1'7 is released.

In order to cut discrete semiconductor elements from the rod 2, asuitable saw blade 18 is provided. If desired, a second saw blade may beemployed to cut from the opposite side simultaneously with the operationof the blade 18. During the spinning motion imparted to the rod 2, theblade 18 may be brought into cutting engagement therewith byconventional feed means (not shown). The upper clutch 1'7 is employed tograsp the rod dining these cut-oh intervals in each cycle. The blade 18may consist of a diamond tipped implement, or other suitable meanscharacterized by sufiicient hardness to provide a clean true cut acrossthe long axis of the crystalline rod.

In severing segments of the rod according to the method of the presentinvention, the required cut-0E period using a single blade was found tobe approximately 1 minute. By employing a pair of blades, as mentionedabove, cutting times of less than thirty seconds are obtainable.

Turning to FIGURE 2, the system for replenishing the supply .of moltensemiconductor material will now be explained. The manner in which thismore or less continuous makeup to the feed crucibles eliminates theprogressive variation in conductivity between the graded junctions hasbeen discussed earlier in this specification.

In FIGURE 2 the successive withdrawal and solidification of thecrystalline material causes the rod 2 to assume the characteristicelongated configuration. In order to compensate or makeup for theresulting depletion in the molten semiconductor material, there isprovided a feedback system which includes a limit switch 19.

The switch 19 includes a pair of condition responsive contacts which arebrought together when the lower clutch 16 reaches its maximum upwarddisplacement. The attainment of this displacement, of course, signifiesthe loss of a predetermined volume of molten material from the feedcrucible.

In order to initiate the required replenishment, the clutch 16 closesthe contacts of limit switch 19. The momentary closure of the contactsof the limit switch results in the closure of the timer switch 20. Thetimer switch 20 may comprise a conventional relay which will close uponreceipt of a current pulse, and remain closed for a predeterminedadjustable time interval.

The timer switch 20 is interposed between the power supply 21 and thevibrator unit 22' of a conventional Syntron vibrator unit 22. Thus,power from the unit 21 is available to energize the Syntron feeder 22only during theinterval that timer switch 20 remains closed.

The Syntron unit is provided with an inclined trough 22 which is mountedto receive a supply of granular semiconductor material from the hopper.The semiconductor material may comprise a mixture of semiconductor anddope in predetermined proportions. When power is supplied to thevibrator unit 22', via the timer switch 26, the resulting vibratorymovement imparted to the trough causes semiconductor material drawnfrornthe hopper 23 to advance along the trough and drop into the charge pot93. It will be appreciated that the charge pot 9A may receivesemiconductor material from the hopper in the same manner. Thisreplenishment process is carried out only during the intervals when thetrough is subjected to vibratory movement by the unit 22.

The quantity of material which is added is carefully calibrated toreplace the volume withdrawn during the crystallization processoccurring at the end of the rod 2. Where the melt-back is accomplishedby raising the liquid level of the molten material, the volume of thematerial which is periodically added is great enough to raise the liquidlevel the required amount. The calibration for such volume is effectedby adjusting the length of time that timer switch 20 allows the powersupply 21 to energize the vibrator unit 22. Thus, by replacing a volumeof material equal to that withdrawn, the temperature and mass of themolten semiconductor material, as well as the growth rate ofcrystallization is maintained substantially constant and continuous.

As an alternative technique, the feed can be arranged to make up moltensemiconductor material as it is used. Thus, in place of adding an amountof semiconductor material after the formation of a predetermined portionof the crystal, it is continuously added through the barrel valves A and1013 as it is used. An arrangement can be employed for this purposewhich either estimates or senses the material used and governs themake-up or which determines or senses or estimates the rate of use andcontrols make-up in response thereto. in fact the make-up can be on aproportional, differential or integral basis or on any combination ofthese.

In practicing the continuous operation of the apparatus-for fabricationtaught by the present invention, a seed crystal is immersed to a smalldepth in the molten melt of semiconductor material. The seed crystal maybe mounted upon a suitable arbor, or mandrel (not shown) which extendsvertically upward for engagement by the clutch to.

The seed crystal, during the initial period, must be withdrawn from themelt at a rate which permits the molten material adherent to the crystalto crystallize as rapidly as it is Withdrawn from the melt.

During this initial period, the rod 2 is gradually elevated by means ofclutch 16 at a linear velocity of 1.68 mils per second, whichcorresponds to a formation of 2.89 grams per minute. It will beappreciated that the rod is simultaneously rotated during this interval.

Following the initial period of combined rotary and rectilineardisplacement, a melt-back is required. This may consist of a descent byclutch 16 which efiects reimmersion to a depth of (say) .010, duringwhich time no upward stress is applied to the crystal. Rotary motion ofthe crystal is continued during the melt-back interval. It will berecalled that the liquid level within the growth crucible may be raisedby .010" in place of reimmersing the rod. It will be appreciated thatregardless of which expedient is used for the melt-back process, thecondition must prevail for a total interval of approximately 30 seconds.

Then, the cycle is resumed, and the gradual upward rotary motion of rod2 is resumed. By continuing in this manner, 10 graded type junctionsweighing about 90 grams were formed in approximately 31 minutes time.During the operation, the supply of pulverulent solid semiconductormaterial in the charge pots 9A and 9B was replenished in the mannerexplained earlier, in order to prevent depletion of the supply of liquidphase semiconductor material, and avoid any undesirable increase in theconcentration of impurities. As earlier explained, this replenishment ofthe supply of molten material minimizes variations in the conductivityof the portions of crystallized semiconductor material between thegraded junctions. Thus at the end of this arbitrary period, 10 junctionshad been formed and the system was in its original condition ready forfurther operation.

From the foregoing detailed description, it will be evident that I havedisclosed my invention in full, clear and concise terms as required bythe statute. However, it will be obvious that various modifications,substitutions and alterations may be made therein without departing inany manner from the spirit and scope of the appended claims.

What is claimed:

1. In combination, an enclosed means for containing a supply of moltensemiconductor material, means for maintaining the supply ofsemiconductor material molten, means to introduce semiconductor materialinto the enclosed means, means to withdraw a solidifying mass from thesurface of said molten semiconductor material and to remove thesolidified mass already withdrawn from the surface of said moltenmaterial from the enclosed means while solidifying mass is beingwithdrawn from the surface of said molten material and sealing meanscooperating with the solidified mass at its point of removal from theenclosed means to seal against gas leakage during removal of thesolidified mass.

2. In combination, an enclosed means containing a supply of moltensemiconductor material, means for maintaining the supply ofsemiconductor material molten, means including a barrel valve tointroduce semiconductor material into the enclosed means, means towithdraw a solidifying mass from the surface of said moltensemiconductor material and to remove the solidified mass from theenclosed means and sealing means cooperating with the solidified mass atits point of removal from the enclosed means to seal against gas leakageduring removal of the solidified mass.

3. In combination, an enclosed means for containing a supply of moltensemiconductor material, means for maintaining the supply ofsemiconductor material molten, means to introduce semiconductor materialinto the enclosed means, means to withdraw a solidifying mass from thesurface of said molten semiconductor material and to remove thesolidified mass already withdrawn from the surface of said moltenmaterial from the enclosed means while solidifying mass is beingwithdrawn from the surface of said molten material, sealing meanscooperating with the solidified mass at its point of removal from theenclosed means to seal against gas leakage during removal of thesolidified mass, and means to cool the solidified mass during removalthereof.

4. In combination, an enclosed means for containing a supply of moltensemiconductor material, means for maintaining the supply ofsemiconductor material molten, means to withdraw a solidifying mass fromthe surface of said molten semiconductor material and to remove thesolidified mass already withdrawn from the surface of said moltenmaterial from the enclosed means while solidifying mass is beingwithdrawn from the surface of said molten material and sealing meanscooperating with the solidified mass at its point of removal from theenclosed means to seal against gas leakage during removal of thesolidified mass.

5. In combination, an enclosed means for containing a supply of moltensemiconductor material, means for maintaining the supply ofsemiconductor material molten, means to withdraw a solidifying mass fromthe surface of said molten semiconductor material and to remove thesolidified mass already withdrawn from the surface of said moltenmaterial from the enclosed means while solidifying mass is beingwithdrawn from the surface of said molten material, sealing meanscooperating with the solidified mass at its point of removal from theenclosed means to seal against gas leakage during removal of thesolidified mass and means to cool the solidified mass during removal.

6. In combination, an enclosed means for containing a supply of moltensemiconductor material, means for maintaining the supply ofsemiconductor material molten, means to withdraw a solidifying mass fromthe surface of said molten semiconductor material and to remove thesolidified mass already withdrawn from the surface of said moltenmaterial from the enclosed means while solidifying mass is beingwithdrawn from the surface of said molten material, sealing meanscooperating with the solidified mass at its point of removal from theenclosed means to seal against gas leakage during removal of thesolidified mass, means to cool the solidified mass ,during removaland-means to cut-ofi the solidified massindiscrete lengths.

maintaining the supply of semiconductor material molten,

a pair of clutches mounted outside the enclosed means to apply tractiveeffort to a column of semiconductor crystal undergoing crystallizationin proximity to the surface of the molten semiconductor material andbeing removed from the enclosed means and sealing means cooperating withthe solidified mass at its point of removal from the enclosed means toseal against gas leakage during removal of the solidified mass.

8. In combination, an enclosed means containing a supply of moltensemiconductor material, means for maintaining the supply ofsemiconductor material molten, means to withdraw a solidifying mass fromthe surface of said molten semiconductor material and to remove thesolidified mass from the enclosed means, sealing means cooperating withthe solidified mass at its point of removal from the enclosed means toseal against gas leakage during removal of the solidified mass and meansto introduce semiconductor material into the enclosed means in responseto removal of the solidified mass from the enclosed means. a

9. In combination, an enclosed .means containing a supply of moltensemiconductor material, means for maintaining the supply ofsemiconductor material molten, means to withdraw a solidifying mass fromthe surface of said molten semiconductor material and to remove thesolidified mass from the enclosed means, sealing means cooperating withthe solidified mass at its point of removal from the enclosed means toseal against gas leakage during removal of the solidified mass and meansto introduce semiconductor material into the enclosed means in an amountcorrelated with removal of the solidified mass from the enclosed means.

References ited in the file of this patent UNITED STATES PATENTS 835,061George Nov. 6, 1906 2,591,304 Schuller Apr. 1, 1952 2,686,212 Horn eta1. Aug. 10, 1954 2,686,864 Wroughton Aug. 17, 1954 2,727,839 SparksDec. 20, 1955 2,727,840 Teal Dec. 20, 1955 2,770,533 Kahmann Nov. 13,1956 2,793,103 Emeis Mar. 21, 1957 2,809,136 Mortimer Oct. 8, 19572,876,147 Kniepkamp Mar. 3, 1959 FOREIGN PATENTS 1,127,036 France Aug.6, 1956

1. IN COMBINATION, AN ENCLOSED MEANS FOR CONTAINING A SUPPLY OF MOLTENSEMICONDUCTOR MATERIAL, MEANS FOR MAINTAINING THE SUPPLY OFSEMICONDUCTOR MATERIAL MOLTEN, MEANS TO INTRODUCE SEMICONDUCTOR MATERIALINTO THE ENCLOSED MEANS, MEANS TO WITHDRAW A SOLIDIFYING MASS FROM THESURFACE OF SAID MOLTEN SEMICONDUCTOR MATERIAL AND TO REMOVE THESOLIDIFIED MASS ALREADY WITHDRAWIN FROM THE SURFACE OF SAID MOLTENMATERIAL FROM THE ENCLOSED MEANS WHILE SOLIDIFYING MASS IS BEINGWITHDRAWN FROM THE SURFACE OF SAID MOLTEN MATERIAL AND SEALING MEANSCOOPERATING WITH THE SOLIDIFIED MASS AT ITS POINT OF REMOVAL FROM THEENCLOSED MEANS TO SEAL AGAINST GAS LEAKAGE DURING REMOVAL OF THESOLIDIFIED MASS.